Re: [RFC PATCH v1 2/2] serial/8250: Avoid getting lock in RT atomic context

[Leonardo Bras <leobras@redhat.com>]

On Tue, Jan 16, 2024 at 08:49:14AM +0100, Jiri Slaby wrote:
> On 16. 01. 24, 8:32, Leonardo Bras wrote:
> > With PREEMPT_RT enabled, a spin_lock_irqsave() becomes a possibly sleeping
> > spin_lock(), without preempt_disable() or irq_disable().
> > 
> > This allows a task T1 to get preempted or interrupted while holding the
> > port->lock. If the preempting task T2 need the lock, spin_lock() code
> > will schedule T1 back until it finishes using the lock, and then go back to
> > T2.
> > 
> > There is an issue if a T1 holding port->lock is interrupted by an
> > IRQ, and this IRQ handler needs to get port->lock for writting (printk):
> > spin_lock() code will try to reschedule the interrupt handler, which is in
> > atomic context, causing a BUG() for trying to reschedule/sleep in atomic
> > context.
> > 
> > So for the case (PREEMPT_RT && in_atomic()) try to get the lock, and if it
> > fails proceed anyway, just like it's done in oops_in_progress case.
> 
> Hmm, that appears incorrect to me.
> 
> Perhaps we need a raw spin lock? Or maybe I am totally off, as my RT
> knowledge is close to zero.

If we have a raw_spin_lock_irqsave() here, it would hurt RT by a lot since 
disabling interrupts is usually bad at the RT kernel, and serial console 
can be used a lot.

> 
> This needs advices from RT folks...

Agree. All help is welcome in this case!

Thanks!
Leo

> 
> > Signed-off-by: Leonardo Bras <leobras@redhat.com>
> > ---
> >   drivers/tty/serial/8250/8250_port.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> > index 8ca061d3bbb92..8480832846319 100644
> > --- a/drivers/tty/serial/8250/8250_port.c
> > +++ b/drivers/tty/serial/8250/8250_port.c
> > @@ -3397,7 +3397,7 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
> >   	touch_nmi_watchdog();
> > -	if (oops_in_progress)
> > +	if (oops_in_progress || (IS_ENABLED(CONFIG_PREEMPT_RT) && in_atomic())
> >   		locked = uart_port_trylock_irqsave(port, &flags);
> >   	else
> >   		uart_port_lock_irqsave(port, &flags);
> 
> -- 
> js
> suse labs
>